The invention relates to scintillation cameras, and more particularly relates to scintillation camera systems which are used to produce a tomographic image. In particular, the invention relates to SPECT tomography of relatively small body organs such as the heart. Reference is made to U.S. Pat. No. 4,774,410.
In conventional tomographic imaging, a focusing collimator is set up so that the focal point is located outside the patient's body, at such a distance that the region of focus is just large enough to include the entire body slice to be imaged. Then, the camera head is rotated 360.degree. around the body to ensure full sampling. Thereafter, each image of the slice is backprojected, and all of the backprojected images are filtered and added together to produce a final tomographic image of the slice.
Annular focusing collimators are known, but in the conventional annular focusing collimator the collimator is made up of a plurality of identical sections, each having its own individual focal point which is located outside the region of interest. Where, for example, an annular collimator has three sections, it may be regarded as three individual camera heads which are mounted together and simultaneously rotated like a multi-head camera. However, in all cases such collimators have a plurality of focal points which during use are all located outside the region of interest.
To avoid artifacts caused by truncation, an image of for example an entire slice through the torso is a necessary by product of any attempt to merely image for example the heart. As a result, a small body organ such as the heart cannot be faithfully imaged without imaging the entire body slice in which the heart is contained.
Since the heart is relatively small as compared with the torso, production of an image of the heart can be accomplished only by a wasteful use of computer resources, which must be used to image the entire slice rather than merely imaging the particular region of interest.
It would be advantageous to provide a scintillation camera system which could produce an image of a relatively small body organ such as the heart without devoting equal effort to imaging other parts of the body which are not of interest.
In accordance with the invention, there is provided a collimator which has the shape of a closed curve. Within each plane of interest the collimator has one and only one focal point. The focal point is located inside the interior region of the collimator. Advantageously, the dimensions of the collimator are chosen such that during use the focal point is positioned inside the region of interest.
One major advantage of this focusing scheme is that the information which appears at the outer periphery of the collimator directly represents a "backprojected" image value (or layergram) which is to be associated with each location of the focal point. Therefore, it is unnecessary to acquire a plurality of images about that point and to backproject them using a computer. For each point, all necessary information is acquired at one time.
Also, because there is essentially complete sampling about the focal point, it is possible to reconstruct an image of an organ such as the heart with a preferentially enhanced signal-to-noise ratio relative to the organ's surroundings. This can be done by moving the focal point in such a manner that it dwells on the organ longer than it dwells on adjacent body regions.
Another advantage of the invention comes about because of the fixed relationship between each point on the scintillator and the focal point. In a conventional SPECT camera a plurality of photodetectors are mounted to a common scintillator and coordinates of a scintillation flash must be located within the scintillator. This is because there is no unique association between points on the scintillator and points in the region of interest; each point on the scintillator receives radiation from many points within the object of interest and the origin of a particular for example gamma ray cannot be determined from the location where it produces a scintillation. However, in the invention, most radiation detected emanates from the focal point, or from locations near it. Furthermore, for each location of the focal point, the information represented by the detected radiation is a weighting of the image values of all of the points in the object.
Since the weighting function itself is determined by the radial ray pattern and the angular beam dispersion of the collimator, as seen from each point on the detector, it follows that for a perfect collimator (i.e. one with an infinitesimally small beam width) the weighting function is inversely proportional to the distance from the focal point, and is not a function of any other variable. Consequently, within each plane of interest, the angular position of a scintillation about the focal point is unimportant. This eliminates the need for much of the weighting circuitry needed to localize a scintillation flash when conventionally focusing collimators are utilized.
Furthermore, the invention makes it possible to increase the speed of the imaging process. There are two reasons for this. First, the collimator completely surrounds the patient and thus receives radiation from a full 360.degree. of arc. Second, the image data is acquired with focal point inside the object of interest. Because the focal point is the point of highest sensitivity, the focal geometry of the invention is more efficient than the focal geometries of know SPECT systems.
In principle, the invention may be regarded as a tomographic imager that works by using techniques normally employed in planar scanners.